With a container molding by blowing pressure action, the tempered preforms of a thermoplastic material, for example preforms of PET (polyethylene terephthalate), are fed to different processing stations inside a blow-molding machine. Typically such a blow-molding machine has a heating apparatus as well as a blow-molding apparatus in whose region the previously tempered preform is expanded by biaxial orientation toward a container. The expansion takes place with the aid of compressed air, which is introduced in a controlled manner into the preform to be expanded. The process engineering sequence of such an expansion of the preform is explained in German published patent specification DE-OS 43 40 291.
The basic construction of a blow-molding station for container molding is described in German published patent specification DE-OS 42 12 583. Possibilities for tempering the preforms are explained in DE-OS 23 52 926.
The preforms as well as the blow-molded containers can be transported with the aid of various handling devices inside the device for blow molding. The use of transport mandrels upon which the preforms can be placed has proven particularly useful. The preforms can, however, also be handled with other carrying devices. The use of gripping tongs for handling preforms and the use of expanding mandrels, which can be introduced for fastening the preform in an opening region, likewise belong to the available constructions.
The already explained handling of the preforms takes place for one in so-called two step processes in which the preforms are first produced in an injection molding process, subsequently subjected to intermediate storage and only later conditioned with respect to their temperature and blow-molded to a container. Secondly, a use in so-called single step methods occurs in which the preforms are suitably tempered immediately after their injection molding production and sufficient solidification and subsequently blow-molding takes place.
Various embodiments are known with respect to the blow-molding stations used. In the case of blowing stations which are arranged on rotating transport wheels, a book-like ability to fold out of the joists can be encountered. Using joists, which can be slid relative to one another or are guided otherwise, are also possible. In stationary blow-molding stations, which are in particular suited for accommodating several cavities for container molding, typically plates arranged parallel to one another are used as joists.
The implementation of a heating of the preforms in connection with a two stage method as well as the corresponding temperature profiling in conducting the one step method as a rule takes place using infrared radiators. With such infrared radiators, relatively high amounts of energy can be introduced into the preforms in a short time.
Conducting a heating of the preforms with high frequency is likewise already known, as a high degree of efficiency can be attained this heating type. A device for implementing high frequency heating is described, for example, in European published patent specification EP-OS 0 849 067.
A typical problem in using heat radiation, especially when using infrared radiation, resides in that a considerable portion of radiation is already converted beneath the surface of the preforms into heat energy, which is then spread further by heat conduction. Owing to the relatively poor heat conductivity of the plastics used, a heat conduction process of this type requires a period of time which can, with the use of high radiation output, lead to causing excessive heating of the surface region of the preform. For this reason, typically cooling air blowers are used at the same time when the preforms are subjected to radiation heating, which produce a surface cooling of the preforms. The concept of cooling air blower here includes not only a use of ambient air, but if need be other suitable gaseous cooling mediums can also be conducted in the direction of the preform. The use of ambient air as cooling air nonetheless has considerable advantages as to cost.
In accordance with a typical process sequence, the preform is provided with a temperature profile in the direction of its long axis. A temperature profile is likewise generated in the peripheral direction of the preform as well when the containers to be blow-molded have special contours. In addition to this, a temperature profile also arises through the wall of the preform proceeding from the outer surface in the direction of the inner surface which is dependent upon the radiation introduced, the effective amount of cooling air, the flow conditions arising and additionally by diverse ambient parameters.
The great number of influencing factors, which change the temperature profile arising between the outer surface and the inner surfaces of the preform, lead to the fact that not all demands for as consistent a production quality can yet be fulfilled. The particular problem in this connection is that the temperature profiles arising have a considerable influence on the material distribution in the blow-molded container as well as on the orientation conditions inside the material of the blow-molded container.
A further influencing factor resides in that, when using a blow-molding machine in a so-called inline operation with direct coupling with a subsequent filling machine, frequently increases and decreases of the production output of the blow-molding machine are necessary. This leads to further unevenness in the manufactured product due to the thermal time constants arising, for example, in heating the machine as well as the heating components themselves.